draw_handler on top of Rendered shading mode
Ok, I'm an idiot.

For future googlers, if you're using POST_PIXEL it should draw on top of all shading modes no problem.

The reason it wasn't working for me was because, hiding in my code, I had:

if context.space_data.shading.type == 'RENDERED':
    return


i.e. at some point in the past I deliberately prevented it from drawing on top of rendered shading mode, probably to be consistent with other overlays in Blender 2.7.